The effects of inter-highlight interval and phase on echolocating bottlenose dolphins performing a two-highlight echo discrimination task

The dolphin's ability to discriminate and classify objects in their environment is a critical feature of its biosonar system. This ability requires the dolphin to extract useful information from complex echoes with differing inter-highlight intervals (IHIs). The current experiment tested the abilities of dolphins to discriminate between simulated echoes with a standard IHI and those with a slightly longer IHI. Seven standard IHIs were tested from 50 to 750 μs. Testing occurred with the two echo highlights in-phase and also with each highlight given a random phase shift. For standard IHIs <300 μs (within the auditory temporal window), thresholds for the in-phase condition were an order of magnitude lower than in the random phase condition. These results are consistent with a reliance on the interaction of the fine structure of the two echo highlights to discriminate small differences in IHI that become "blurred" for the random phase condition, resulting in elevated thresholds. For standard IHIs >300 μs (outside the temporal window), thresholds were higher than those for standard IHIs within the temporal window, and were similar between the two phase conditions, suggesting the use of timing differences between the highlight envelopes.